1. Field of the Invention
The present invention relates to the removal of metal vapor and submicron metal particles from combustion off-gas.
2. The Prior Art
Emissions of toxic metals, specifically, Cd, Pb, Hg, Ni, Sb, As, Ba, Be, Cr, Co, Mn, Se, Ag and Tl, into the air are regulated in the U.S. by the Resource Conservation and Recovery Act ("RCRA") and by the Clean Air Act.
Many wastes contain toxic metal constituents. Often, when these metals are associated with organic and aqueous components, incineration may be the preferred method of waste treatment and disposal. Incineration technologies can be effective in reducing waste volume and destroying organic elements. However, incineration cannot destroy the elemental metal constituents, although high temperature combustion environments will induce metal transformations. These transformations are usually thought to exacerbate their harmful effects, since many of the metal species formed readily vaporize within combustion environments, which vapor will nucleate and condense downstream of the flame, forming a fume of submicron aerosol. These particles, because of their small size, are difficult to collect in pollution control systems. Moreover, combustion gas in such incinerators often contains significant amounts of chlorine as Cl.sub.2 and/or chloride compounds, the presence of which inhibits capture and removal of the metals. Chlorinated metal species that are collected often exhibit increased water leachability.
Using a downflow laboratory combustor, Scotto et al found that lead could be reactively scavenged in-situ by kaolinite powder which was injected into the postflame. See Scotto, M. V. Peterson, T. W., and Wendt, J.O.L. Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, pp. 1109-1117. Reactive scavenging (chemisorption) of a metal occurs at temperatures above the metal vapor dewpoint. Scotto et al also found that, although 99% of the lead could be captured by kaolinite, by reaction forming a lead aluminosilicate, the process was inhibited by the presence of chlorine.
Uberoi and Shadman investigated the use of kaolinite, bauxite, and limestone as sorbents to capture lead and cadmium. Their subcombustion temperature, fixed bed experiments, suggested that crystalline kaolinite might be somewhat less effective than crystalline bauxite in capturing cadmium because of pore closure observed in the cadmium/kaolinite system but not in the cadmium/bauxite system. For the lead/kaolinite system, the formation of a melt on the kaolinite surface appeared to enhance lead capture as lead aluminosilicate. Presence of a melt was also noted in the above-mentioned experiments of Scotto et al. Uberoi and Shadman also found that limestone was not an effective sorbent for either cadmium or lead.
Thus, previous research in this area has focused on the reaction of toxic metals and crystalline sorbents to form stable reaction products. Additionally, previous work has focused on low temperature applications such as those conditions common to flue gas cleaning environments. This has the disadvantage of limiting application to metal/sorbent systems where stable reaction products exist and limiting kinetic rates due to low temperatures. Previous research has been concerned not to expose sorbents to combustor conditions which would sinter or close pores and reduce effective surface area. One mechanism involves reaction between metal vapor and a sorbent crystalline surface, as in the reaction between cadmium and bauxite. Here, large pores prevent pore plugging by reaction products. This mechanism was identified by Uberoi and Shadman. See Uberoi, M., and Shadman, F., AIChE Journal, 36(2):307-309 (1990) and Uberoi, M., and Shadman, F., Environ. Sci. Technol., 25(7):1285-1289 (1991). However, metal sorption on crystalline sorbent surfaces is diminished when pores are plugged, as with cadmium/kaolinite, at moderately high temperatures.
Kubin et al--U.S. Pat. No. 5,092,254 discloses the injection of calcium based sorbents into a combustor for control of acid gases. In this process, great care is taken to avoid sorbent melting, in order to allow acid gas capture to take place. The reaction which is exploited in this process is one between acid gas constituents and crystalline surfaces, contained within a solid particle.